The present invention relates to a camera status display device.
FIG. 22 of the accompanying drawings schematically illustrates a conventional electronic camera circuit. The electronic camera circuit has a power supply 1 composed of a battery for supplying a voltage of 3V which is converted by a regulator 2 into a constant voltage of 1.5V, which is applied to a CMOS LSI circuit 3 and a backup capacitor 4. The CMOS LSI circuit 3 is responsive to input signals supplied from various components in the camera for enabling a liquid crystal display 5 to display various camera statuses. When a main switch 6 is turned on, the voltage from the power supply 1 is applied to an automatic focusing/exposure control unit 7. When a stroboscopic flash switch 8 is turned on, the voltage of the power supply 1 is applied to a stroboscopic flash 9. The flash signal from the stroboscopic flash switch 8 is fed through a resistor 21 to the CMOS LSI circuit 3. The camera circuit also includes a motor circuit composed of a film winding switch 10, two ganged film rewinding switches 11, 12, a film detecting switch 13, and a motor 14, the motor circuit being devised by the present inventor. Signals delivered from the terminals of the motor 14 and indicative of the rotation and its direction of the motor 14 are fed through resistors 22, 23 to the CMOS LSI circuit 3. When the film is to be wound, the film winding switch 10 is turned on to rotate the motor 14 in a normal direction for winding the film and charging a shutter mechanism. When the film is to be rewound, the film detecting switch 13 and the rewinding switches 11, 12 are turned on to reverse the motor for rewinding the film. The CMOS LSI circuit 3 has protective diodes 15 through 20 connected between input terminals and power supply terminals, and common terminals. The regulator 2 serves to control the current flowing in one direction for generating the constant voltage of 1.5V. When the battery 1 is used up, the CMOS LSI circuit 3 is energized by the backup capacitor 4.
In the illustrated electronic camera circuit, the power supply voltage for the CMOS LSI circuit 3 is 1.5V, whereas the input voltage for the CMOS LSI circuit 3 is 3V. Since the regulator 2 for producing the constant voltage of 1.5V generally allows the current to flow in only one direction, but fails to absorb a current in the opposite direction, the input signal voltage for the CMOS LSI circuit 3 is higher than the power supply voltage for the CMOS LSI circuit 3, thus charging the backup capacitor 4 through the protective diodes 15-17 to increase the power supply voltage for the CMOS LSI circuit 3. For example, when the terminal of the motor 14 coupled to the rewinding switch 11 goes high in level upon normal rotation of the motor 14, a current I1 flows back through the protective diode 15 to the power supply for the CMOS LSI circuit 3. If the current I1 is larger than a current I2 consumed by the CMOS LSI circuit 3, then an excess current (I1-I2) charges the backup capacitor 4 to increase the power supply voltage for the CMOS LSI 3. Where I2=2 microamperes and the resistor 22 has a resistance of 0.5 Megohm, then the current I1 is approximately 4.6 microamperes.
One solution to the above problem is to slice each input signal voltage for the CMOS LSI circuit 3 with a zener diode 24 for preventing the current I2 from flowing back, as shown in FIG. 23. However, this solution is disadvantageous in that a current I3 flowing through the zener diode 24 is wasteful, and the zener diode 24 required to be connected exteriorly to the CMOS LSI circuit 3 results in an increased cost and a greater number of parts required.
FIG. 24 shows another solution in which the level of each input signal voltage for the CMOS LSI circuit 3 is shifted by a zener diode 25 for preventing the current I2 from flowing back. The problem with this design is that when the input signal voltage varies to a lower level, the input signal voltage for the CMOS LSI circuit 3 cannot be kept at a high level, and use of the zener diode 25 increases the cost.